1. Field of the Invention
The present invention relates to a diversity antenna, and to a wireless communication apparatus employing it.
2. Description of the Prior Art
In recent years, with the advancement of communication technology, wireless communication apparatuses have come to be made compact. One type of small-size antenna used in such wireless communication apparatuses is the inverted-F antenna. The inverted-F antenna uses an element of which the length equals xc2xc of the wavelength, and permits the feed point to be located at the center of the element. Thus, the inverted-F antenna is suitable for miniaturization. By using two such inverted-F antennas, it is possible to build a diversity antenna.
FIG. 7 is an external perspective view of a conventional diversity antenna. The conventional diversity antenna 50 is composed of a plate-shaped inverted-F antenna 60 and a plate-shaped inverted-F antenna 70. The plate-shaped inverted-F antennas 60 and 70 are arranged on the top surface of a chassis 51 of a wireless communication apparatus.
The inverted-F antenna 60 is composed of a plate 61, a grounding plate 62, a feed wire 63, and a feed point 64. The inverted-F antenna 70 is composed of a plate 71, a grounding plate 72, a feed wire 73, and a feed point 74.
The plates 61 and 71 are each formed as a rectangular metal conductor, the lengths of the sides of which are determined according to the frequency used. The grounding plates 62 and 72 are also metal conductors, which serve to ground the plates 61 and 71, respectively, to the chassis 51 of the wireless communication apparatus. The feed wires 63 and 73 feed ultra-high-frequency current to the feed points 64 and 74 to excite the plates 61 and 71, respectively. The feed points 64 and 74 are where the largest amount of current can be fed to the plates 61 and 71, respectively. The chassis 51 of the wireless communication apparatus is box-shaped.
In FIG. 7, the plane that is parallel to the top surface of the chassis 51 of the wireless communication apparatus and on which the inverted-F antennas 60 and 70 are arranged is called the horizontal plate H, and the direction perpendicular to the top surface of the chassis 51 of the wireless communication apparatus is called the vertical axis V.
FIGS. 8A and 8B are diagrams showing the directivity patterns of the conventional diversity antenna 50 shown in FIG. 7 and described above. FIG. 8A is a diagram showing the directivity patterns of the inverted-F antenna 60 with respect to the vertical axis V, where the directivity pattern for vertically polarized radio waves is indicated with a solid line 80 and the directivity pattern for horizontally polarized radio waves is indicated with a broken line 81. FIG. 8B is a diagram showing the directivity patterns of the inverted-F antenna 70 with respect to the vertical axis V, where the directivity pattern for vertically polarized radio waves is indicated with a solid line 82 and the directivity pattern for horizontally polarized radio waves is indicated with a broken line 83.
The conventional structure described above, however, has the following disadvantages. The directivity patterns of the inverted-F antennas 60 and 70 with respect to the vertical axis V shown in FIGS. 8A and 8B clearly show the following. First, for vertically polarized radio waves, the inverted-F antennas 60 and 70 exhibit lower gains below the top surface of the chassis 51 of the wireless communication apparatus on which they are arranged than above that surface, and have null points in the direction of the vertical axis V. Second, the inverted-F antennas 60 and 70 exhibit lower gains for horizontally polarized radio waves as indicated with broken lines than for vertically polarized radio waves as indicated with solid lines. Thus, combining antennas of this type to build a diversity antenna does not help to overcome low gains in particular directions or on particular polarization planes. Such a diversity antenna may operate satisfactorily in applications where the system employing it is used in a fixed state or position and requires transmission and reception of radio waves polarized in a particular way, but not where the system is used in an unpredictable state or position and requires transmission and reception of radio waves polarized in any way and traveling in and from any direction.
An object of the present invention is to provide a diversity antenna free from the disadvantages mentioned above, and -to provide a wireless communication apparatus employing such a diversity antenna.
To achieve the above object, according to the present invention, the following improvements are made. The first and second antennas no longer have, in their directivity patterns with respect to the vertical axis V, null points in the direction of the vertical axis V as are observed conventionally. Thus, the first and second antennas exhibit gains in all directions, and are thus largely non-directional. Moreover, lower gains for horizontally polarized radio waves than for vertically polarized radio waves as are observed conventionally are improved. In diversity operation, the improved gains of the first and second antennas for horizontally polarized radio waves make it possible to cope satisfactorily with radio waves polarized on different polarization planes.
Specifically, according to the present invention, a diversity antenna is provided with: a first inverted-F antenna composed of a first elongate conductor portion, a first grounding conductor portion formed at one side of the first elongate conductor portion so as to be substantially perpendicular to the first elongate conductor portion, and a first feeding conductor portion formed at another side of the first elongate conductor portion so as to be substantially perpendicular to the first elongate conductor portion; and a second inverted-F antenna composed of a second elongate conductor portion, a second grounding conductor portion formed at one side of the second elongate conductor portion so as to be substantially perpendicular to the second elongate conductor portion, and a second feeding conductor portion formed at another side of the second elongate conductor portion so as to be substantially perpendicular to the second elongate conductor portion. The first and second inverted-F antennas are arranged so that the center axes of the first and second elongate conductor portions are substantially perpendicular to each other and that the center axes of the first and second feeding conductor portions are substantially parallel to each other.
Thus, according to the present invention, it is possible to reduce the differences between the gains for vertically polarized radio waves and the gain for horizontally polarized radio waves. This makes it possible to realize a diversity antenna that copes with both vertically and horizontally polarized radio waves.
According to the present invention, the first inverted-F antenna has a first printed circuit board, the first grounding conductor portion is electrically connected to the ground pattern of the first printed circuit board, and the first feeding conductor portion is electrically connected to the feed point of the first printed circuit board. On the other hand, the second inverted-F antenna has a second printed circuit board, the second grounding conductor portion is electrically connected to the ground pattern of the second printed circuit board, and the first and second printed circuit boards are arranged substantially parallel so as to face each other.
Thus, according to the present invention, the first and second antennas have no null points in the direction of the vertical axis V for either vertically or horizontally polarized radio waves, and therefore exhibit gains in all directions, i.e., are largely non-directional. In addition, the first and second antennas are arranged so that their center axes are perpendicular to each other. This makes it possible, in diversity operation, to cope satisfactorily with radio waves polarized on different polarization planes.
According to the present invention, the first and second printed circuit boards each have a perimeter equal to the wavelength of the radio waves used.
Thus, according to the present invention, the radio waves reach, by diffraction, as far as the surfaces of the first and second printed circuit boards opposite to the surfaces thereof on which the first and second elongate conductor portions are formed. This makes the directivity patterns of the first and second inverted-F antennas closer to non-directional, and thus further reduces the difference between the gain for vertically polarized radio waves and the gain for horizontally polarized radio waves.
According to the present invention, with respect to the center about which the first and second printed circuit boards are arranged so as to face each other, the surface of the first printed circuit board on which the first elongate conductor portion is formed and the surface of the second printed circuit board on which the second elongate conductor portion is formed face away from each other.
Thus, according to the present invention, even if an obstacle or the like is located between the first printed circuit board of the first inverted-F antenna and the second printed circuit board of the second inverted-F antenna, it does not seriously affect the operation of the diversity antenna.
According to the present invention, the first elongate conductor portion of the first inverted-F antenna is arranged so that the center axis thereof is substantially vertical, with the first feeding conductor portion up, and the second elongate conductor portion of the second inverted-F antenna is arranged so that the center axis thereof is substantially horizontal.
Thus, according to the present invention, the center axes of the first and second inverted-F antennas coincide with the polarization planes of vertically and horizontally polarized radio waves. This makes it possible, in diversity operation, to cope satisfactorily with radio waves polarized on different polarized planes. Moreover, the center axis of the first inverted-F antenna is arranged vertically with the first feeding conductor portion, which is connected to the feed point at which the largest amount of current flows, located up. This reduces the effects on the diversity antenna of an obstacle located under the first inverted-F antenna. That is, even when a system employing those antennas is installed on a wall or desk, its effects can be reduced.
According to the present invention, the second inverted-F antenna is arranged so that the second elongate conductor portion is located above the horizontal center axis of the second printed circuit board.
Thus, according to the present invention, the second elongate conductor portion is located above the horizontal center axis of the second printed circuit board. This reduces the effects on the diversity antenna of an obstacle located under the first inverted-F antenna. That is, even when a system employing those antennas is installed on a wall or desk, its effects can be reduced.
According to the present invention, in a wireless communication apparatus including a diversity antenna as described above and a transmitter/receiver circuit board unit connected thereto, the first and second inverted-F antennas are arranged substantially symmetrically about the transmitter/receiver circuit board unit placed between them.
Thus, according to the present invention, it is possible, inside the miniaturized wireless communication apparatus, to secure a sufficient interval between the first and second inverted-F antennas and arrange the transmitter/receiver circuit board unit between the first and second inverted-F antennas. This makes it possible to realize a wireless communication apparatus employing a diversity antenna of which the operation is not seriously affected by the transmitter/receiver circuit board unit.